Process for forming nonwoven webs from combined filaments

ABSTRACT

In a process for forming nonwoven webs from combined binder and matrix filaments, both types of filaments are separately charged to obtain uniform filament separation and the binder filaments are neutralized prior to combining with the matrix filaments. The combined filaments are forwarded by means of a jet for collection as nonwoven web. Neutralizing the charge on the binder filaments allows a higher charge level to be used on the matrix filaments to improve product uniformity.

United States Patent (1 1 m1 3,71 1,898 Debbas i 51 Jan. 23, 1973 [5 1 PROCESS FOR FORMING NONWOVEN 3,384,944 5/1968 Medeiros et al ..28/] SM ux WEBS FROM COMBINED FILAMENTS 3,402,227 9/1968 Knee ..264/24 [75] lnventor: Sflrnir Costandi Debbas, Hendersong 3332i l vine, Tenn. [73] Assignee: E. I. du Pont de Nemours and Co., Primary Examiner-Robert R. Mackey Wilmington, Del. Attorney-Howard P. West, Jr.

[22] Filed: April 13, 1971 Appl. No.: 133,487

References Cited UNITED STATES PATENTS 8/1967 Kinney ..264/24 [57] ABSTRACT 5 Claims, 5 Drawing Figures YE o H PATENTEUJAH 23 1915 Y I I0 INVENTOR i SAM IR COSTANDI DEBBAS 3 BY A MM w ATTORNEY PATENTEDJMI 23 I975 3.71 l. 898

SHEET 2 0r 2 INVENTOR SAN IR COSTANDI DEBBAS W mwyq ATTORNEY PROCESS FOR FORMING NONWOVEN WEBS FROM COMBINED FILAMENTS BACKGROUND OF THE INVENTION u. s. Pat. No. 3,338,992 describes a process for the I production of continuous filament nonwoven fabrics in which multifilament strands of continuous filaments under tension are electrostatically charged and forwarded by means of a jet device toward a laydown zone. The tension on the filaments is released as they exit the jet device thereby permitting them to separate due to the repelling effect of the electrostatic charge and the filaments are collected as a nonwoven web.

The fabric weight uniformity of the nonwoven sheet produced by the above process generally improves as the charge level on the filaments is increased. However, with any jet device, a point of diminishing return is reached when further increases in charge level cause the filaments to become attracted to the jet walls. When this occurs, the filaments tend to hang from the jet resulting in laydown defects. When the nonwoven web is produced from two types of filaments (for example, higher melting matrix filaments and lower melting binder filaments), means must be provided to blend the two types of filaments so as to obtain a uniform distribution of the binder filaments throughout the matrix filaments of the nonwoven web. Lack of uniform dis tribution of the binder contributes to poor uniformity of the sheet as well as inefficient utilization of the binder.

U. S. Pat. No. 3,384,944 describes a method for blending two moving ribbons of filaments of different widths wherein a series of convex guides is used to divert the narrow ribbon to a given width, a series of concave guides is used to converge the wider ribbon to the same width and both ribbons are then combined on a cylindrical surface and forwarded toward a receiving zone to form a nonwoven web. An alternate method for controlling the width of a moving ribbon of filaments is described in U. S. Defensive Publication No. T 864009, published July 15, 1969. Here, the ribbon of filaments is engaged by a fan-shaped comb and a guide mounted below the comb, the ribbon width being varied by relative movement between the comb and the guide.

While the above methods can be relied upon in certain cases to control the width of the moving ribbon of filaments, there is a limit to how much increase in width can be obtained with purely mechanical means. Further, the uniformity of separation of the filaments of the binder ribbon is not as good as might be desirable, because of a tendency for the binder filaments to form discrete bundles of two or more filaments which remain together as the matrix and binder ribbons are combined. As a result, the binder filaments remain bunched in the nonwoven web leading to poor utilization of the binder and to a poorer sheet uniformity.

One solution to the bunching problem might be to increase the electrostatic charge on the binder filaments in order to enhance the filament separation. Attempts to do so, however, have shown that at the levels of charge needed to obtain the desired filament separation, the binder filaments tend to migrate to the edges of the swath being laid down, as described in U. S. Pat. 3,436,797, thus forming a nonwoven web with nonuniform distribution of binder throughout its thickness. This migration of the binder filaments to the edges of the swath also increases the tendency for these filaments to become attracted to the jet walls thereby causing jet hangs. When hot air is used in the jet devices to relax the matrix filaments, contact of the binder filaments with hot jet walls may cause them to melt and form spits, i.e., small drops of molten binder material which are blown onto the nonwoven web, thus, forming another type of defect. This action may also cause plugging of the jet thereby resulting in discontinuities and yield losses.

It has surprisingly been found that if the ribbon of binder filaments is electrostatically charged to a sufficiently high level to obtain uniform filament separation and is then neutralized before being combined with the matrix filaments ribbon, the improved separation between the binder filaments is retained through the laydown step and results in improved binder distribution in the nonwoven web. Furthermore, because the limiting charge'level for a given jet (i.e., the charge at which jet hangs occur) depends on the total charge on the filaments which is essentially equal to the sum of the charges on the individual ribbons, neutralizing the charge on the binder filaments makes it possible to increase the charge level on the matrix filaments by an amount equivalent to that which would otherwise be on the binder filaments, thereby resulting in an over-all improvement in the nonwoven web fabric weight uniformity.

One object of this invention is to provide a process for obtaining a nonwoven sheet having an improved distribution of two types of filaments. Another object is to provide a process for obtaining a nonwoven web having an improved fabric weight uniformity.

SUMMARY OF THE INVENTION These and other objects are obtained in a process for preparing a nonwoven web comprising forwarding two bundles of substantially parallel continuous filaments, one bundle having a relatively large number of filaments and the other bundle having relatively fewer filaments, guiding the bundles in the form of ribbons through separate corona charging zones to form electrostatically charged filament ribbons of essentially the same width, neutralizing the charge on the filaments in the ribbon containing the fewer filaments, combining the ribbons, forwarding the combined ribbons by means of a jet device toward a receiver and collecting the filaments in a tensionless state on the receiver as a nonwoven web.

In this process, the bundle of filaments having a larger number of filaments will generally be composed of higher melting matrix filaments and it may be formed into a ribbon using the methods described in the aforementioned U. S. Pat. No. 3,384,944 or U. S. Defensive Publication No. T 864009. The bundle of filaments containing relatively fewer filaments will generally be composed of the lower melting binder filaments, whose width may also be controlled by the aforementioned methods. Preferably, however, the width of the binder ribbon is controlled by first applying an electrostatic charge which will cause it to diverge to more than the desired final width, then snubbing this ribbon with appropriate guides to bring it to the desired width before neutralizing the charge and combining it with the ribbon of matrix filaments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of an apparatus for spinning filaments from two spinnerets and uniformly blending the filaments prior to web laydown.

FIG. 2 is a view along line 2-2 of FIG. 1.

FIG. 3 is a view along line 3-3 of FIG. 1.

FIG. 4 is a partial view along line 44 of FIG. 1.

FIG. 5 shows an alternative arrangement for guiding the bundle containing fewer filaments.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Referring to FIGS. 1, 2 and 3, matrix filaments l are spun from spinneret 2 and quenched by radial quench diffuser 3. The filaments are brought together in ribbon form on concave guide 4 which, in combination with concave guides 5 and 8, converges the ribbon of filaments to the width required for passage through jet device 10. Draw rolls 9 provide the tension to attenuate the filaments in the region between the spinneret and concave guide 4.

The filaments are electrostatically charged by means of corona discharge devices comprising target bars 11 and charging heads 7. The ribbon of filaments is passed into light contact with slowly rotating target bars 11 which are positioned adjacent charging heads 7 which have needle or wire electrodes. As described in U. S. Pat. No. 3,163,753, a corona discharge is generated by applying a high electric potential to the electrodes and grounding the target bars. A cylindrical snub guide 6 may be used to provide further control and stabilization of the rapidly moving ribbon of filaments.

Binder filaments 12 (generally fewer in number) are spun from spinneret l3 and quenched by cross flow air using chimney 14. The filaments pass through a fanshaped comb l5 and contact cylindrical guide 16. Relative movement of the guide 16 with respect to comb 15 changes the depth of the filaments within the fanshaped comb and alters the width of the ribbon accordingly. This can be accomplished as indicated in FIG. 1 either by changing the position of guide 16 longitudinally with respect to the dents of comb 15 while maintaining comb 15 in a fixed position or by fixing the position of guide 16 and pivotally moving comb 15 toward or away from guide 16 or both as long as relative movement between comb l5 and guide 16 is obtainable. An electrostatic charge is applied using the corona device 7 and grounded target bar 11 and the width of the ribbon is stabilized by passage over a grooved guide 17. The ribbon of binder filaments is diverged to essentially the same width as the ribbon of matrix filaments leaving the concave guide 8 and is then neutralized by passage between two sets of needle electrodes 18 and 19, one of which is grounded and the other is connected to a high positive DC power supply. The two ribbons are then brought together and uniformly blended on the first draw roll 9. Binder filaments of an 80/20 copolymer of polyethylene terephthalate/polyethylene isophthalate are suitable for use with polyethylene terephthalate matrix filaments. A binder content within the range of 2 to 25 percent of the web weight may be used.

As in the case of the matrix filaments l, the tension required to attenuate the binder filaments and to move them over the guide and target bar of the corona discharge device is provided by the draw rolls 9. Jet device 10 provides the tension needed to strip the composite filament ribbon from the last draw roll and to forward the filaments toward a web laydown zone of a foraminous belt receiver (not shown). Wide nonwoven sheets may be prepared by overlapping the output from several jets as described in U. S. Pat. No. 3,402,227.

FIG. 4 indicates the relative positions of the binder filaments 12, the fan-shaped comb l5 and the cylindrical guide 16. Typically, the wider side of the comb guide indicated by the numeral 21 may contain a dent spacing of 10 dents per inch, while the narrower side 22 may have a spacing of 20 dents per inch. Movement of guide 16 longitudinally with respect to the dents decreases or increases (depending on direction of movement) the width of the ribbon as desired.

The comb guide 15 is adapted to be inserted into the filaments as they leave the spinneret. It is mounted on a mechanism 26 which is slidably mounted on rod 27 (FIG. 1) itself attached in a suitable manner to the spinning machine. The comb is pivotably attached to the mechanism 26 so that it can be rotated from the horizontal to the vertical position, i.e., toward and away from guide 16. During startup, the comb guide is placed in the vertical position and moved to within about one inch of the spinneret 13. It is then rotated into the filaments and moved down and fixed at the desired position above the cylindrical guide 16.

FIG. 5 shows an alternative arrangement for guiding the binder filament ribbon in which the grooved guide 17 has been replaced by two smooth-surfaced guides 23 and 24. Guide 23 is preferably an 18 inch radius concave guide mounted so that it may swivel about its longitudinal axis. Guide 24 is convex or cylindrical in shape. Proper placement of these guides permits snubbing of the ribbon to control its width as well as lateral movement to provide registry with the matrix filament ribbon. This arrangement will also operate without comb 15 when the quench air flow direction is filaments of polyethylene terephthalate and binder filaments of a copolyester consisting of 79/2l mole percent of polyethylene terephthalate/polyethylene isophthalate.

The binder distribution isjudged subjectively by contrast dyeing of the binder. A number of samples are contrast dyed using a solution of Du Pont Oil Blue A dye (Color Index Solvent Blue 36) in a trichloroethylene bath. The web sample is submerged in the dye solution (48 grams in 1,000 ml of trichloroethylene) for 20-40 seconds. The sample is allowed to drain for approximately 1 minute and blotted on a bed of paper towels. After the sample is dried, it is rinsed in a perchloroethylene bath for about 30 seconds, then after allowing the rinse to drain, it is rinsed again in a second perchloroethylene bath, drained and dried. This treatment dyes the binder filaments blue but does not affect the matrix filaments which remain white. The samples are placed side-byside and a visual judgment on the binder distribution is made. The judgment is based on how well the binder is dispersed among the matrix filaments and whether or not there are binder filaments in the feather edges of each swath.

A subjective evaluation of the presence of binder on the web surfaces is obtained from samples dyed as above, placed between two pieces of white cardboard and pressed in a Pasadena press at about 250C. Under these conditions, the dyed binder softens and transfers an impression to the cardboard sheets. Evaluation of the binder distribution throughout the thickness of the web is obtained by first delaminating the samples into several layers and then carrying out the contrast dyeing procedure.

The fabric weight uniformity of the samples is measured using a beta-ray gauge adapted to measure the fabric weight of an area corresponding to a 1-inch diameter circle and mounted on a feeding device adapted to advance the samples between measurements so that the centers of adjacent circles are spaced I inch apart. The web sample, with any feather edge removed, is placed in the feeding device and 60 fabric weight measurements are made along a line parallel to the machine direction of the sample. The sample is then moved laterally 1 inch and 60 more measurements are taken. The procedure is repeated until a total of ten lines have been scanned (i.e., 600 individual fabric weight measurements/sample). The variance of the 60 measurements along each of the ten lines is then calculated and then an average is computed from these individual variances. Finally, an over-all coefficient of variation is calculated from the equation:

Percent CV:

w/ average variance X100 average of 600 fabric wt. measurements The filament charge data are relative measurements obtained bypassing the charged filaments between two rotating vane electric field transducers which measure the potential on the filament ribbon. The transducers comprise two ungrounded metal plates, each having a rotating grounded shutter positioned so that in the presence of charged filaments passing by the plates the shutters alternately expose the plates to the charged filaments and to ground. In this manner, the plates charge and discharge generating an alternating voltage which is proportional to the electrostatic charge on the filaments. In some cases, these charge data are converted into absolute measurements in units of microcoulombs/meter or microcoulombs/gram. This conversion is based on a calibration of the instrument derived by collecting filaments in a Faraday Cage.

EXAMPLE I Polyethylene terephthalate matrix filaments and poly(ethylene terephthalate/isophthalate) (79/21) were melt spun at 280 C. through multihole spinnerets (406 matrix filaments and 68 binder filaments) at throughputs of 65.6 and 9.4 lb/hr., respectively. The apparatus used was similar to that represented in FIGS.

1 to 4, withdraw rolls operating at 3,000 yds./minut e using a 5 inch wide slot jet of the type described in U. S. Pat. No. 3,302,237 and fitted with a diffuser section of the type described in U. S. Pat. No. 3,325,906.

Nonwoven webs were collected at different charge levels with and without neutralization of the binder charge as well as with partial neutralization of the charge. In each case, the charge level was measured as hereinbefore described. Some tests were run with air at ambient temperature in the jet and other tests with 250 C. temperature air to provide relaxation of the matrix filaments.

In all cases, the binder separation and distribution in the nonwoven web were significantly improved when the charge on the binder filaments was 10 volts (i.e., with complete, partial or without neutralization) when compared to a charge of 4 volts. However, with 10 volts of charge on the binder and a total charge of 80 volts on the combined bundles, poor process operability was observed in the form of spits or jet hangs, when the neutralizer was not used. The spits or hangs were eliminated when the neutralizer was turned on. The maximum operable charge (i.e., no spits or hangs) without neutralization was determined as 66 volts on the matrix filaments and 10 volts on the binder filaments (i.e., a total of 76 volts on the combined ribbons), versus 80 volts on the combined ribbon with neutralization (i.e., 80 volts on the matrix filaments and 10 volts on the binder filaments which were subsequently neutralized to give a net total charge of 80 volts on the combined ribbons). As shown in Table I, webs produced with neutralization had 9l2 percent improvement in fabric weight uniformity, at the same total charge level, when compared with those produced without neutralization.

TABLE I Fabric Wt.

Jet Air Total Net Charge On Uniformity Temp. C. Charge Binder Neutralized %CV Volts Volts* Ambient 80 I0 Yes 7.8 80 I0 No 8.6

80 IO partially to 4 v. 8.5

250 I0 Yes 7.3 70 10 No 8.3

" 70 IO partially to 4 v. 8.0

Positive corona charge applied to matrix and binder ribbons EXAMPLE II Nonwoven webs were prepared as in Example I except that the draw roll speed was 3,100 ypm and a jet air temperature of 235 C. was used.

TEST I In a first test, using the apparatus of FIGS. 1-4 but without binder ribbon neutralization, a positive charge of 0.5 microcoulombs/gram was applied onto the binder filament ribbon and the positive charge on the matrix filaments was slowly increased to the maximum operable level before spitting occurred. A nonwoven web was collected under these conditions and analyzed for fabric weight uniformity and binder distribution.

TEST 2 The above procedure was repeated using the binder guiding system of FIG. 5, but without binder ribbon neutralization, this time applying a positive charge of 1.5 microcoulombs/gram to the binder filaments. A web sample was again collected at the maximum operable charge level.

TEST 3 I distribution.

TABLE II w I mm Maximum Fabric operable weight charge level uni- Charge on on combined Binder Binder fonnity, Test binder, bbons, neutralidistripercent number re/g. [JG-lg. zation butlon CV 1 0.5 2.9 No Poor 9.5 2 1.5 2.3 No Good..." 9.3 3 1.6 3.2 Yes .-do 7.8

EXAMPLE lll Nonwoven webs were prepared using the same spinning conditions as in Example I but with the binder guiding arrangement of FIG. 5 and using a 7 inch wide slot jet. it was found that, due to arcing at the corona charging device, it was not possible to get sufficient copolymer ribbon width (about 6.5 inches) with a posi- 8 ti ve charge. Using negative charging, sufficient chargecould be applied to obtain the desired ribbon width before arcing occurred. Again, an improvement in nonwoven web fabric weight uniformity was noted when the charge on the copolymer ribbon was neutralized before combining it with the matrix filament ribbon.

What is claimed is:

1. In a process for preparing a nonwoven web including the steps of forwarding two bundles of substantially parallel continuous filaments in the form of ribbons of essentially the same width through separate charging zones, one of said ribbons having a relatively large number of filaments, the other of said ribbons having relatively fewer filaments, electrostatically charging each of said ribbons, combining said ribbons and forwarding the combined ribbons by means of a jet for collection as a web, the improvement comprising, neutralizing the charge on said other ribbon prior to combining said ribbons.

2. The process as defined in claim 1, said bundles being of different polymer compositions.

3. The process as defined in claim 2, said separate charging zones being corona charging zones.

4. The process as defined in claim 1, the charge on said other ribbon being neutralized by passing said other ribbon between two sets of opposed needle electrodes, one of said sets being connected to a positive high potential source, the other of said sets being grounded.

5. The process as defined in claim 1, including the additional steps of allowing said other charged ribbon to diverge to a width greater than said same width, then guiding said other ribbon back to said same width prior to neutralizing said other ribbon. 

2. The process as defined in claim 1, said bundles being of different polymer compositions.
 3. The process as defined in claim 2, said separate charging zones being corona charging zones.
 4. The process as defined in claim 1, the charge on said other ribbon being neutralized by passing said other ribbon between two sets of opposed needle electrodes, one of said sets being connected to a positive high potential source, the other of said sets being grounded.
 5. The process as defined in claim 1, including the additional steps of allowing said other charged ribbon to diverge to a width greater than said same width, then guiding said other ribbon back to said same width prior to neutralizing said other ribbon. 